For various types of printers, such as an inkjet printer, etc., using a recording head (hereinafter referred to simply as a head), there are a so-called multi-pass method for making recording while alternately moving a head and a recording medium in a nearly orthogonal direction by using short heads, and a so-called one-pass method for making recording by relatively moving a recording medium in a direction nearly orthogonal to a line head with the use of the long head (line head) having a length that can cover the entire region of one side of the recording medium. The latter has a structure where one head comprises many recording elements although its operations are simple and recording can be made at high speed. Therefore, this method has a disadvantage that a yield at the time of head manufacturing is low, or a cost becomes high.
Accordingly, a method for configuring a pseudo line head (hereinafter referred to as a “head unit” depending on need) by arranging a plurality of short heads, in which recording elements are arranged in one direction (main scanning direction), by displacing the short heads to prevent their positions from interfering with one another is known as recited in Patent Document 1.
At this time, recording corresponding to image data can be made by switching the input image data in an overlapping region of the recordable regions of a plurality of recording heads, which are alternately arranged (staggered) in parallel to main scanning direction, and by displacing the recording timing of each of the heads by D/V according to a displacement D in a conveyance direction (sub-scanning direction) and a conveyance velocity V.
The above described pseudo line head implemented by combining a plurality of short heads has the advantages of both the multi-pass method and the one-pass method. However, unless the relative positions, the inclinations, and the recording timings of the short heads are suitably adjusted, stripe density unevenness, a white patch, a discontinuous line, or a color shift occurs at a joint of a recording image, leading to possible loss of recording quality.
Accordingly, the technique of Patent Document 1 recites a method for improving stripe density unevenness and a white patch, which occur at a joint of a recording image, by arranging adjacent short heads in predetermined positions, and by selecting a recording element to be used according to the interval (projection interval in sub-scanning direction) of recording elements at a joint.
Additionally, as a method for improving a yield at the time of head manufacturing, which is determined by variations in nozzle positions of a long head, Patent Document 2 recites a method for absorbing variations in nozzle positions at the time of manufacturing in a paper feed direction by delaying ink emission timing independently of each nozzle in a song head.
However, a mechanism for adjusting recording timing for each recording element (nozzle) is complicated, and an increase in cost is unavoidable also in consideration of a head replacement at the time of breakage.
In the meantime, also if a pseudo line head is configured by combining short heads, there is a problem that a discontinuity or a color shift at a joint of a recording image occurs when the distortion or the inclination shift of each short head, variations in arrangement positions of recording elements due to manufacturing unevenness, and the like occur.
For example, if an inclination shift occurs in a short head that configures a head unit as shown in FIG. 1, and if recording timing is made to simply match at the center of a recording pattern, this causes a problem such that a level difference occurs at a joint, and a linear line is recorded discontinuously. This is because the number of recording element sequences actually reaches several hundreds, and a shift is prone to occur at the end of a head even if the head has almost no inclination, although FIG. 1 is schematically depicted. Additionally, for a short head having an inclination shift or distortion, a color shift can sometimes occur in superposed color recording as shown in FIG. 2.
Here, the quality of an image to be recorded is evaluated with linearity in sub-scanning direction (conveyance direction of a recording medium), continuity at a head boundary, a color shift in superposed color recording, and the like. However, quality to be stressed may differ by image to be recorded. For example, if an image composed of a lot of drawn lines is recorded at many times, stress is placed on continuity at a head boundary. Or, for an image the color hue of which is important, stress is placed on a color shift in superposed color recording.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2002-144542    Patent Document 2: Japanese Published Unexamined Patent Application No. 2000-62148